In the manufacture of semiconductor devices, ion implantation is often used to dope semiconductor workpieces (e.g., silicon wafers) with impurities. For example, ion implanters or ion implantation systems may treat a workpiece with an ion beam to produce n-type or p-type doped regions or to form passivation layers on the workpiece. When used for doping semiconductors, the ion implantation system may inject a selected ion species to produce a desired extrinsic material.
In general, in ion implantation systems, maintaining an ion beam having charge neutrality (e.g., forming an ion beam plasma having a substantially equal density of both positively charged and negatively charged particles) is important for producing a high quality ion implantation. Charge neutrality aids in producing uniformity of an ion beam by preventing beam blow up (e.g., the tendency of the like charged ions comprising a beam to repel each other, thereby causing the beam to diverge away from its intended path), which can cause beam current loss and non-uniformities. If these non-uniformities are not corrected the workpiece may be implanted non-uniformly and wafer yield (i.e. the ratio of wafer area implanted with desired properties to total wafer area) may suffer.
Also, during ion implantation of a semiconductor wafer charge may build up on electrodes insulated by a gate oxide from the bulk semiconductor. This effect, commonly referred to as charging, can be detrimental to the semiconductor circuit if the voltages of the insulated feature (e.g. the gate electrode) exceed the breakdown voltage of the insulator (e.g. the gate oxide) such that resultant damage to the gate oxide occurs. It can be appreciated that the charging rate and voltage increase with beam current, and that ion implantation with ever increasing beam currents represents an increasing processing challenge.
To counteract the beam blow up and the charging problems, charging of an ion beam can be compensated for by providing electric charge of the opposite sign to the beamline and/or workpiece to be implanted. For example, for a positive ion beam it is common practice to provide electrons in an amount equal to the amount of ions per unit time to the workpiece (i.e., to match the ion beam current with an equal electron current to the workpiece). This compensation is typically brought about by devices such as plasma electron floods (PEF) apparatus that are configured to generate electrons (e.g., via electron generating processes such as thermionic emission, secondary emission, or discharge) and direct the generated electrons to the beamline and/or workpiece. Therefore, a plasma electron flood apparatus is an important subsystem of ion implanters that provides neutralizing charged particles into a beamline comprising an ion beam to generate an ion beam having an overall charge neutrality.